Self-cleaning synthetic body and method for producing the same

ABSTRACT

The invention relates to self-cleaning synthetic bodies, obtained as follows: a) a siloxane coating (a) is applied to a synthetic substrate and cured, b) the polar fraction of the surface energy of the cured siloxane coating on the substrate is increased to a value of at least 10 mN/m and c) a coating (b) containing photocatalytically active TiO2 particles is applied to the synthetic substrate and cured.

The present invention relates to self-cleaning plastics articles whichhave siloxane coatings with TiO₂ particles.

Self-cleaning articles become superhydrophilic through irradiation withUV light in the presence of water, and are capable of breaking downorganic contamination to give carbon dioxide and water. This property ofthe surface is generally achieved via the photocatalytic effect oftitanium dioxide, which can be fixed to solid supports and, for example,securely bonded to the substrate by stoving at high temperatures. Anexample is provided by silicate glasses for self-cleaning windows, asdescribed by Rhodia Chemie in EP 850 203 B1.

For esthetic reasons, plastics substrates, e.g. acrylic sheet orpolycarbonate widely used as glazing material or for transparent noisebarriers are intended to have a maximum of transparency and cleanliness,in order to permit clear visibility of the surrounding landscape forrailway passengers or drivers of automobiles. They are especially usedon bridges, but also to relieve the monotony of concrete noise barriers,and are intended to contribute to a reduction in the fatigue experiencedby drivers of automobiles.

Automotive exhaust gases, abraded tire material, dust and organiccontamination are causes of rapid deterioration in the attractivenessand esthetic properties of transparent noise barriers. Numerous attemptshave therefore been made to equip transparent plastics withself-cleaning coatings. The aim here has been to make the photocatalyticactivity of titanium dioxide available for decomposing the contaminationadhering to the substrate surface, but on the other hand to protect theorganic substrate itself from damage due to the titanium dioxide.

Plastics articles provided with self-cleaning siloxane coatings arelikewise known. These substrates usually have a double layer of siloxanewith different composition, only the outer layer comprising aphotocatalytic additive, such as the anatase or brookite form of TiO₂.

By way of example, the Publication EP-A-1 022 318 describes coatedsheets of plastic which have a photocatalytic layer. However, theexamples merely give sheets or films with a thin coating totaling 1.2μm, and these thin coatings have only very low scratch resistance.

Although the description says that it is also possible to obtain thickerlayers, the only indication given is that thicker layers can be obtainedvia repeated application of siloxane coating compositions. However,without the use of additives the TiO₂-containing siloxane layer lacksadhesion to the siloxane layer initially applied, which serves as primerto protect the underlying plastics article.

In order to solve the problem of insufficient substrate adhesion, usemay be made of inorganic-organic layers composed of siloxane networks,as the layer isolating the plastics substrate. Given a suitablecomposition, the adhesion of the layers is markedly better than that ofpurely inorganic material, and the hybrid character of the layers makesthem more resistant than purely organic layers to the photocatalyticactivity of the titanium dioxide.

However, experiments have shown that weathering, in particular UVirradiation, reduces, over the course of time, the scratch resistance ofplastics articles provided with inorganic-organic layers, thus reducingthe transparency of the plastics articles. In addition, the scratchresistance immediately after production is also unsatisfactory.

Problems with these plastics articles of the prior art are thereforetheir low scratch resistance or their low weathering resistance. Theresult is that environmental effects cause ablation of the coating overthe course of time, and they therefore lose their self-cleaningcapability.

In the light of the prior art stated and discussed herein, it wastherefore an object of the present invention to provide self-cleaningplastics articles which have particularly high scratch resistance.

Another object of the invention consisted in plastics articles with highdurability, in particular high resistance to weathering or UVirradiation.

Another object underlying the invention was to providescratch-resistant, self-cleaning plastics articles which are capable ofparticularly simple production. For example, the substrates which can beused to produce the plastics articles should in particular be thoseobtainable via extrusion or injection molding, or else via castingprocesses.

It was moreover therefore an object of the present invention to provideplastics articles capable of production at low cost.

Another object of the present invention consisted in providingscratch-resistant, self-cleaning plastics articles which have excellentmechanical properties. This property is particularly important forapplications where the plastics article is to have high resistance toimpact.

The plastics articles should moreover have particularly good opticalproperties.

Another object of the present invention consisted in providing plasticsarticles whose size and shape can easily be adapted to requirements.

The plastics articles described in claim 1 achieve these objects andalso achieve other objects which, although they are not specificallymentioned, are obvious or necessary consequences of the circumstancesdiscussed herein. Useful modifications of the plastics articles of theinvention are protected by the subclaims dependent on claim 1.

Claim 22 achieves the underlying object in relation to a productionprocess.

Self-cleaning plastics articles which have particularly high scratchresistance are successfully provided by taking a plastics substrate and

-   a) applying and curing a siloxane coating (a),-   b) increasing the polar component of the surface energy of the cured    siloxane coating to a value of at least 10 mN/m and-   c) applying and curing a coating (b) comprising photocatalytic TiO₂    particles.

Particular advantages, inter alia, achieved by the measures of theinvention are the following:

-   -   The plastics articles of the present invention are highly        resistant to surface scratching.    -   The plastics articles of the invention have high resistance to        UV irradiation.    -   Plastics articles moreover exhibit a particularly high level of        self-cleaning even when the level of UV irradiation is very low.    -   The plastics articles of the present invention can moreover be        produced at particularly low cost, with no need to use expensive        additives.    -   The present invention moreover permits the production of        self-cleaning coatings on plastics substrates previously coated        with siloxanes. A particular advantage of this is that it is        possible to take sheets from the production of plastics articles        provided with scratch-resistant coatings and then subsequently        provide these sheets with another coating which has        self-cleaning properties.    -   The scratch-resistant plastics articles of the present invention        may be adapted to certain requirements. In particular, the size        and the shape of the plastics article may be varied within a        wide range, with no resultant impairment of its scratch        resistance or self-cleaning property. Furthermore, the present        invention also provides plastics articles with excellent optical        properties.    -   The scratch-resistant plastics articles of the present invention        have good mechanical properties.

The plastics articles of the invention are obtainable through coating ofplastics substrates. Suitable plastics substrates for the purposes ofthe present invention are known per se. These substrates encompass inparticular polycarbonates, polystyrenes, polyesters, such aspolyethylene terephthalate (PET) and polybutylene terephthalate (PBT),cycloolefinic polymers (COC), and/or poly(meth)acrylates. Preference isgiven here to polycarbonates, cycloolefinic polymers andpoly(meth)acrylates, particular preference being given topoly(meth)acrylates.

Polycarbonates are known to persons skilled in the art. Polycarbonatesmay be formally regarded as polyesters composed of carbonic acid and ofaliphatic or aromatic dihydroxy compounds. They are easily obtainable byreacting diglycols or bisphenols with phosgene or carbonic diesters inpolycondensation or transesterification reactions.

Preference is given here to polycarbonates which derive from bisphenols.Particular bisphenols among these are 2,2-bis(4-hydroxyphenyl)propane(bisphenol A), 2,2-bis-(4-hydroxyphenyl)butane (bisphenol B),1,1-bis-(4-hydroxyphenyl)cyclohexane (bisphenol C),2,2′-methylenediphenol (bisphenol F),2,2-bis(3,5-dibromo-4-hydroxyphenyl)propane (tetrabromobisphenol A) and2,2′-bis(3,5-dimethyl-4-hydroxyphenyl)propane (tetramethyl-bisphenol A).

These aromatic polycarbonates are usually prepared by interfacialpolycondensation or by transesterification, details being given inEncycl. Polym. Sci. Engng. 11, 648-718.

In interfacial polycondensation, the bisphenols in the form of aqueousalkaline solution are emulsified in inert organic solvents, such asmethylene chloride, chlorobenzene or tetrahydrofuran, and reacted withphosgene in a reaction involving stages. Amines are used as catalysts,and phase-transfer catalysts are used in the case of sterically hinderedbisphenols. The resultant polymers are soluble in the organic solventsused.

The properties of the polymers may be varied widely via the selection ofthe bisphenols. If different bisphenols are used together, blockpolymers can also be constructed in multistage polycondensations.

Cycloolefinic polymers are polymers obtainable by using cyclic olefins,in particular by using polycyclic olefins.

Cyclic olefins encompass, for example, monocyclic olefins, such ascyclopentene, cyclopentadiene, cyclohexene, cycloheptene, cyclooctene,and also alkyl derivatives of these monocyclic olefins having from 1 to3 carbon atoms, examples being methyl, ethyl or propyl, e.g.methylcyclohexene or dimethylcyclohexene, and also acrylate and/ormethacrylate derivatives of these monocyclic compounds. Furthermore,cycloalkanes having olefinic side chains may also be used as cyclicolefins, an example being cyclopentyl methacrylate.

Preference is given to bridged polycyclic olefin compounds. Thesepolycyclic olefin compounds may have the double bond either in the ring,in which case they are bridged polycyclic cycloalkenes, or else in sidechains. In that case they are vinyl derivatives, allyloxycarboxyderivatives or (meth)acryloxy derivatives of polycyclic cycloalkanecompounds. These compounds may also have alkyl, aryl or aralkylsubstituents.

Without any intended resultant restriction, examples of polycycliccompounds are bicyclo[2.2.1]hept-2-ene (norbornene),bicyclo[2.2.1]hept-2,5-diene (2,5-norbornadiene),ethylbicyclo[2.2.1]hept-2-ene (ethylnorbornene),ethylidenebicyclo[2.2.1]hept-2-ene (ethylidene-2-norbornene),phenylbicyclo[2.2.1]hept-2-ene, bicyclo[4.3.0]nona-3,8-diene,tricyclo[4.3.0.1^(2,5)]-3-decene,tricyclo[4.3.0.1^(2,5)]-3,8-decene-(3,8-dihydrodicyclopentadiene),tricyclo[4.4.0.1^(2,5)]-3-undecene, tetracyclo[4.4.0.1^(2,5),1^(7,10)]-3-dodecene,ethylidenetetracyclo[4.4.0.1^(2,5).1^(7,10)]-3-dodecene,methyloxycarbonyl-tetracyclo[4.4.0.1^(2,5), 1^(7,10)]-3-dodecene,ethylidene-9-ethyltetracyclo[4.4.0.1^(2,5), 1^(7,10)]-3-dodecene,pentacyclo-[4.7.0.1^(2,5), O, O^(3,13), 1^(9,12)]-3-pentadecene,pentacyclo-[6.1.1^(3,6).0^(2,7).0^(9,13)]-4-pentadecene,hexacyclo-[6.6.1.1^(3,6).1^(10,13).0^(2,7).0^(9,14)]-4-heptadecene,dimethylhexacyclo[6.6.1.1^(3,6).1^(10,13).0^(2,7).0^(9,14)]-4-heptadecene,bis-(allyloxycarboxy)tricyclo[4.3.0.1^(2,5)]decane,bis(meth-acryloxy)tricyclo[4.3.0.1^(2,5)]decane,bis(acryloxy)tricyclo[4.3.0.1^(2,5)]decane.

The cycloolefinic polymers are prepared using at least one of thecycloolefinic compounds described above, in particular the polycyclichydrocarbon compounds. The preparation of the cycloolefinic polymersmay, furthermore, use other olefins which can be copolymerized with theabovementioned cycloolefinic monomers. Examples of these are ethylene,propylene, isoprene, butadiene, methylpentene, styrene, andvinyltoluene.

Most of the abovementioned olefins, and in particular the cycloolefinsand polycycloolefins, may be obtained commercially. Many cyclic andpolycyclic olefins are moreover obtainable by Diels-Alder additionreactions.

The cycloolefinic polymers may be prepared in a known manner, as set outinter alia in the Japanese Patent Specifications 11818/1972, 43412/1983,1442/1986 and 19761/1987 and in the published Japanese PatentApplications Nos. 75700/1975, 129434/1980, 127728/1983, 168708/1985,271308/1986, 221118/1988 and 180976/1990 and in the European PatentApplications EP-A-0 6 610 851, EP-A-0 6 485 893, EP-A-0 6 407 870 andEP-A-0 6 688 801.

The cycloolefinic polymers may, for example, be polymerized in asolvent, using aluminum compounds, vanadium compounds, tungstencompounds or boron compounds as catalyst.

It is assumed that, depending on the conditions, in particular on thecatalyst used, the polymerization can proceed with ring-opening or withopening of the double bond.

It is also possible to obtain cycloolefinic polymers by free-radicalpolymerization, using light or an initiator as free-radical generator.This applies in particular to the acryloyl derivatives of thecycloolefins and/or cycloalkanes. This type of polymerization may takeplace either in solution or else in bulk.

Another preferred plastics substrate encompasses poly(meth)acrylates.These polymers are generally obtained by free-radical polymerization ofmixtures which comprise (meth)acrylates. The term (meth)acryl-atesencompasses methacrylates and acrylates, and also mixtures of the two.

These monomers are well known. Among them are, inter alia,(meth)acrylates derived from saturated alcohols, e.g.methyl(meth)acrylate, ethyl(meth)acrylate, propyl(meth)acrylate,n-butyl(meth)acrylate, tert-butyl (meth)acrylate, pentyl(meth)acrylateand 2-ethylhexyl(meth)acrylate; (meth)acrylates derived from unsaturatedalcohols, e.g. oleyl(meth)acrylate, 2-propynyl(meth)acrylate,allyl(meth)acrylate, vinyl (meth)acrylate;

aryl(meth)acrylates, such as benzyl(meth)acrylate orphenyl(meth)acrylate, where each of the aryl radicals may beunsubstituted or have up to four substituents;

cycloalkyl(meth)acrylates, such as 3-vinylcyclohexyl (meth)acrylate,bornyl(meth)acrylate;

hydroxyalkyl(meth)acrylates, such as 3-hydroxypropyl(meth)acrylate,3,4-dihydroxybutyl(meth)acrylate, 2-hydroxyethyl(meth)acrylate,2-hydroxypropyl(meth)-acrylate;

glycol di(meth)acrylates, such as 1,4-butanediol di(meth)acrylate,(meth)acrylates of ether alcohols, such astetrahydrofurfuryl(meth)acrylate, vinyloxyethoxyethyl (meth)acrylate;

amides and nitriles of (meth)acrylic acid, such asN-(3-dimethylaminopropyl)(meth)acrylamide,N-(diethylphosphono)(meth)acrylamide,1-methacryloylamido-2-methyl-2-propanol;

sulfur-containing methacrylates, such asmethylsulfinylethyl(meth)acrylate, 4-thiocyanatobutyl(meth)acrylate,ethylsulfonylethyl(meth)acrylate, thiocyanatomethyl(meth)acrylate,methylsulfinylmethyl(meth)acrylate, bis((meth)acryloyloxyethyl)sulfide;

multifunctional (meth)acrylates, such as trimethyloylpropanetri(meth)acrylate, pentaerythritol tetra(meth)acrylate andpentaerythritol tri(meth)acrylate.

In one preferred aspect of the present invention, these mixturescomprise at least 40% by weight, preferably at least 60% by weight, andparticularly preferably at least 80% by weight, of methyl methacrylate,based on the weight of monomers.

Besides the (meth)acrylates set out above, the compositions to bepolymerized may also comprise other unsaturated monomers which arecopolymerizable with methyl methacrylate and with the above-mentioned(meth)acrylates.

Examples of these are 1-alkenes, such as 1-hexene, 1-heptene; branchedalkenes, such as vinylcyclohexane, 3,3-dimethyl-1-propene,3-methyl-1-diisobutylene, 4-methyl-1-pentene;

acrylonitrile; vinyl esters, such as vinyl acetate; styrene, substitutedstyrenes having one alkyl substituent in the side chain, e.g.α-methylstyrene and α-ethylstyrene, substituted styrenes having onealkyl substituent on the ring, e.g. vinyltoluene and p-methylstyrene,halogenated styrenes, such as monochlorostyrenes, dichlorostyrenes,tribromostyrenes, and tetrabromostyrenes;

heterocyclic vinyl compounds, such as 2-vinylpyridine, 3-vinylpyridine,2-methyl-5-vinylpyridine, 3-ethyl-4-vinylpyridine,2,3-dimethyl-5-vinylpyridine, vinylpyrimidine, vinylpiperidine,9-vinylcarbazole, 3-vinylcarbazole, 4-vinylcarbazole, 1-vinylimidazole,2-methyl-1-vinylimidazole, N-vinylpyrrolidone, 2-vinylpyrrolidone,N-vinylpyrrolidine, 3-vinylpyrrolidine, N-vinylcaprolactam,N-vinylbutyrolactam, vinyloxolane, vinylfuran, vinylthiophene,vinylthiolane, vinylthiazoles, and hydrogenated vinylthiazoles,vinyloxazoles and hydrogenated vinyloxazoles;

vinyl and isoprenyl ethers;

maleic acid derivatives, such as maleic anhydride, methylmaleicanhydride, maleimide, methylmaleimide; and dienes, such asdivinylbenzene.

The amount generally used of these comonomers is from 0 to 60% byweight, preferably from 0 to 40% by weight, and particularly preferablyfrom 0 to 20% by weight, based on the weight of the monomers, and thecompounds here may be used individually or as a mixture.

The polymerization is generally initiated by known free-radicalinitiators. Examples of preferred initiators are the azo initiators wellknown to persons skilled in the art, e.g. AIBN and1,1-azobiscyclohexanecarbonitrile, and also peroxy compounds, such asmethyl ethyl ketone peroxide, acetylacetone peroxide, dilauryl peroxide,tert-butyl per-2-ethylhexanoate, ketone peroxide, methyl isobutyl ketoneperoxide, cyclohexanone peroxide, dibenzoyl peroxide, tert-butylperoxybenzoate, tert-butylperoxy isopropyl carbonate,2,5-bis(2-ethylhexanoylperoxy)-2,5-dimethylhexane, tert-butylperoxy2-ethylhexanoate, tert-butylperoxy 3,5,5-trimethylhexanoate, dicumylperoxide, 1,1-bis(tert-butylperoxy)cyclohexane,1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, cumylhydroperoxide, 1,1-bis(tert-butyl hydroperoxide,bis(4-tert-butyl-cyclohexyl) peroxydicarbonate, mixtures of two or moreof the abovementioned compounds with one another, and also mixtures ofthe abovementioned compounds with compounds not mentioned which canlikewise form free radicals.

The amount often used of these compounds is from 0.01 to 10% by weight,preferably from 0.5 to 3% by weight, based on the weight of themonomers.

The abovementioned polymers may be used individually or as a mixture.Use may also be made here of various polycarbonates, poly(meth)acrylatesor cycloolefinic polymers which differ, for example, in molecular weightor in monomer composition.

The plastics substrates of the invention may, for example, be producedfrom molding compositions of the abovementioned polymers. For this, useis generally made of thermoplastic shaping processes, such as extrusionor injection molding.

The weight-average molar mass M_(w) of the homo- and/or copolymers to beused according to the invention as molding compositions for producingthe plastics substrates may vary within a wide range, the molar massusually being matched to the application and the method used forprocessing the molding composition. However, with no intended resultantrestriction, it is generally in the range from 20 000 to 1 000 000g/mol, preferably from 50 000 to 500 000 g/mol, and particularlypreferably from 80 000 to 300 000 g/mol.

The plastics substrates may also be produced by cell casting processes.In these, by way of example, suitable (meth)acrylic mixtures are chargedto a mold and polymerized. These (meth)acrylic mixtures generallycomprise the (meth)acrylates set out above, in particular methylmethacrylate. The (meth)acrylic mixtures may moreover comprise thecopolymers set out above, and also, in particular for viscosityadjustment, may comprise polymers, in particular poly(meth)acrylates.

The molding compositions used to produce the plastics substrates, andalso the acrylic resins, may also comprise conventional additives of anytype. Examples of these are antistatic agents, antioxidants,mold-release agents, flame retardants, lubricants, dyes, flow improvers,fillers, light stabilizers and organophosphorus compounds, such asphosphites, phosphorinanes, phospholanes or phosphonates, pigments,weathering stabilizers and plasticizers. However, the amount ofadditives is restricted in relation to the application.

Particularly preferred molding compositions which encompasspoly(meth)acrylates are obtainable commercially with the trade namePLEXIGLAS® from Degussa AG. Preferred molding compositions whichencompass cycloolefinic polymers may be purchased with the trade name®Topas from Ticona and ®Zeonex from Nippon Zeon. Polycarbonate moldingcompositions are obtainable, by way of example, with the trade name®Makrolon from Bayer or ®Lexan from General Electric.

The plastics substrate particularly preferably encompasses at least 80%by weight, in particular at least 90% by weight, based on the totalweight of the substrate, of poly(meth)acrylates, polycarbonates and/orcycloolefinic polymers. The plastics substrates are particularlypreferably composed of polymethyl methacrylate, and this polymethylmethacrylate may comprise conventional additives.

In one preferred embodiment, plastics substrates may have an impactstrength to ISO 179/1 of at least 10 kJ/m², preferably at least 15kJ/m².

The shape, and also the size, of the plastics substrate are notimportant for the present invention. Substrates generally used oftenhave the shape of a sheet or a panel, and have a thickness in the rangefrom 1 mm to 200 mm, in particular from 5 to 30 mm.

The plastics articles of the present invention are first provided with asiloxane coating which protects the plastics substrate fromphotocatalytic degradation due to the photocatalytically-acting coveringlayer.

Scratch-resistant siloxane lacquers which can be used to produce thecoating (a) are known per se, and are used on polymeric glazingmaterials. Their inorganic character gives them good resistance to UVradiation and to weathering. By way of example, the production of theselacquers is described in EP-A-0 073911. Conventional lacquers are, interalia, those which comprise water and/or alcohol as solvent, besides thesiloxane condensation products.

These siloxane lacquers may be obtained, inter alia, via condensation orhydrolysis of organosilicon compounds of the general formula (I)R¹ _(n)SiX_(4-n)  (I),where R¹ is a group having from 1 to 20 carbon atoms, X is an alkoxyradical having from 1 to 20 carbon atoms, or a halogen, and n is aninteger from 0 to 3, and where the various radicals X or R¹ may in eachcase be identical or different.

The expression “a group having from 1 to 20 carbon atoms” characterizesradicals of organic compounds having from 1 to 20 carbon atoms. Itencompasses alkyl groups, cycloalkyl groups, aromatic groups, alkenylgroups and alkynyl groups having from 1 to 20 carbon atoms, and alsoheteroaliphatic and heteroaromatic groups which have in particularoxygen atoms, nitrogen atoms, sulfur atoms and phosphorus atoms, besidescarbon atoms and hydrogen atoms. These groups mentioned may be branchedor unbranched, and the radical R¹ here may be substituted orunsubstituted. Among the substituents are in particular halogens, groupshaving from 1 to 20 carbon atoms, nitro groups, sulfonic acid groups,alkoxy groups, cycloalkoxy groups, alkanoyl groups, alkoxycarbonylgroups, sulfonic ester groups, sulfinic acid groups, sulfinic estergroups, thiol groups, cyanide groups, epoxy groups, (meth)acryloylgroups, amino groups and hydroxy groups. For the purposes of the presentinvention, the term “halogen” means a fluorine atom, chlorine atom,bromine atom or iodine atom.

Among the preferred alkyl groups are the methyl, ethyl, propyl,isopropyl, 1-butyl, 2-butyl, 2-methylpropyl, tert-butyl, pentyl,2-methylbutyl, 1,1-dimethylpropyl, hexyl, heptyl, octyl,1,1,3,3-tetramethylbutyl, nonyl, 1-decyl, 2-decyl, undecyl, dodecyl,pentadecyl group, and the eicosyl group.

Examples of preferred cycloalkyl groups are the cyclopropyl, cyclobutyl,cyclopentyl, cyclohexyl and cycloheptyl group, and the cyclooctyl group,these having substitution, where appropriate, by branched or unbranchedalkyl groups.

Among the preferred alkenyl groups are the vinyl, allyl,2-methyl-2-propene, 2-butenyl, 2-pentenyl, 2-decenyl group, and the2-eicosenyl group.

Among the preferred alkynyl groups are the ethynyl, propargyl,2-methyl-2-propyne, 2-butynyl, 2-pentynyl group, and the 2-decynylgroup.

Among the preferred alkanoyl groups are the formyl, acetyl, propionyl,2-methylpropionyl, butyryl, valeroyl, pivaloyl, hexanoyl, decanoylgroup, and the dodecanoyl group.

Among the preferred alkoxycarbonyl groups are the methoxycarbonyl,ethoxycarbonyl, propoxycarbonyl, butoxycarbonyl, tert-butoxycarbonyl,hexyloxycarbonyl, 2-methylhexyloxycarbonyl, or decyloxycarbonyl group,or dodecyloxycarbonyl group.

Among the preferred alkoxy groups are the methoxy, ethoxy, propoxy,butoxy, tert-butoxy, hexyloxy, 2-methylhexyloxy, or decyloxy group, ordodecyloxy group.

Examples of preferred cycloalkoxy groups are cycloalkoxy groups whosehydrocarbon radical is one of the abovementioned preferred cycloalkylgroups.

Among the preferred heteroaliphatic groups are the abovementionedpreferred cycloalkyl radicals in which at least one carbon unit has beenreplaced by O, S or an NR⁸ group, R⁸ being hydrogen or an alkyl grouphaving from 1 to 6 carbon atoms, or being an alkoxy or aryl group havingfrom 1 to 6 carbon atoms.

According to the invention, aromatic groups are radicals of one orpolynuclear aromatic compounds preferably having from 6 to 14 carbonatoms, in particular from 6 to 12 carbon atoms. Heteroaromatic groupsare aryl radicals in which at least one CH group has been replaced by N,and/or at least two adjacent CH groups have been replaced by S, NH or O.According to the invention, preferred aromatic or heteroaromatic groupsderive from benzene, naphthalene, biphenyl, diphenyl ether,diphenylmethane, diphenyldimethylmethane, bisphenone, diphenyl, sulfone,thiophene, furan, pyrrole, thiazole, oxazole, imidazole, isothiazole,isoxazole, pyrazole, 1,3,4-oxadiazole, 2,5-diphenyl-1,3,4-oxadiazole,1,3,4-thiadiazole, 1,3,4-triazole, 2,5-diphenyl-1,3,4-triazole,1,2,5-triphenyl-1,3,4-triazole, 1,2,4-oxadiazole, 1,2,4-thiadiazole,1,2,4-triazole, 1,2,3-triazole, 1,2,3,4-tetrazole, benzo[b]thiophene,benzo[b]furan, indole, benzo[c]thiophene, benzo[c]furan, isoindole,benzoxazole, benzothiazole, benzimidazole, benzisoxazole,benzisothiazole, benzopyrazole, benzothiadiazole, benzotriazole,dibenzofuran, dibenzothiophene, carbazole, pyridine, pyrazine,pyrimidine, pyridazine, 1,3,5-triazine, 1,2,4-triazine,1,2,4,5-triazine, quinoline, isoquinoline, quinoxaline, quinazoline,cinnoline, 1,8-naphthyridine, 1,5-naphthyridine, 1,6-naphthyridine,1,7-naphthyridine, phthalazine, pyridopyrimidine, purine, pteridine, or4H-quinolizine, diphenyl ether, anthracene and phenanthrene.

Preferred radicals R¹ can be represented by the formulae (II),—(CH₂)_(m)NH—[(CH₂)_(n)—NH]_(p)H  (II),where m and n are numbers from 1 to 6, and p is zero or one,or the formula (III)

where q is a number from 1 to 6,or the formula (IV)

where R₁ is methyl or hydrogen and r is a number from 1 to 6.

The radical R¹ is very particularly preferably a methyl or ethyl group.

In relation to the definition of the group X in formula (I) in respectof the alkoxy group having from 1 to 20 carbon atoms and also of thehalogen, reference may be made to the abovementioned definition, wherethe alkyl radical of the alkoxy group may likewise preferably berepresented by the formulae (II), (III) or (IV) set out above. The groupX preferably represents a methoxy or ethoxy radical or a bromine orchlorine atom.

These compounds may be used individually or as a mixture to preparesiloxane lacquers.

Depending on the number of the halogens or on the number of alkoxygroups bonded via oxygen to silicon, hydrolysis or condensation formschains or branched siloxanes from the silane compounds of the formula(I). It is preferable for at least 60% by weight, in particular at least80% by weight, of the silane compounds used to have at least threealkoxy groups or halogen atoms, based on the weight of the condensablesilanes.

Tetraalkoxysilanes encompass tetramethoxysilane, tetraethoxysilane,tetra-n-propoxysilane, tetraisopropoxysilane and tetra-n-butoxysilanes;

trialkoxysilanes encompass methyltrimethoxysilane,methyltriethoxysilane, ethyltrimethoxysilane, n-propyl-trimethoxysilane,n-propyltriethoxysilane, isopropyl-triethoxysilane,isopropyltrimethoxysilane, isopropyltripropoxysilane,n-butyltrimethoxysilane, n-butyltriethoxysilane,n-pentyltrimethoxysilane, n-hexyltrimethoxysilane,n-heptyltrimethoxysilane, n-octyltrimethoxysilane,vinyltrimethoxysilane, vinyltrimethoxysilane,cyclohexyltrimethoxysilane, cyclohexyltriethoxysilane,phenyltrimethoxysilane, 3-chloropropyltrimethoxysilane,3-chloropropyltriethoxysilane, 3,3,3-trifluoropropyltrimethoxysilane,3,3,3-trifluoropropyltriethoxysilane, 3-aminopropyltrimethoxysilane,3-aminopropyltrimethoxysilane, 2-hydroxyethyltriethoxysilane,2-hydroxyethyltriethoxysilane, 2-hydroxypropyltrimethoxysilane,2-hydroxypropyltriethoxysilane, 3-hydroxypropyltrimethoxysilane,3-hydroxypropyltriethoxysilane, 3-mercaptopropyltrimethoxysilane,3-mercaptopropyltrimethoxysilane, 3-isocyanatopropyltrimethoxysilane,3-isocyanatopropyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane,3-glycidoxypropyltriethoxysilane,2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane,2-(3,4-epoxycyclohexyl)ethyltriethoxysilane,3-(meth)acryloxypropyltrimethoxysilane,3-(meth)acryloxypropyltriethoxysilane, 3-ureidopropyltrimethoxysilaneand 3-ureidopropyltriethoxysilane;

dialkoxysilanes encompass dimethyldimethoxysilane,dimethyldiethoxysilane, diethyldimethoxysilane, diethyldiethoxysilane,di-n-propyldimethoxysilane, din-propyldiethoxysilane,diisopropyldimethoxysilane, diisopropyldiethoxysilane,di-n-butyldimethoxysilane, di-n-butyldiethoxysilane,di-n-pentyldimethoxysilane, di-n-pentyldiethoxysilane,di-n-hexyldimethoxysilane, di-n-hexyldiethoxysilane,di-n-peptyldimethoxysilane, di-n-peptyldiethoxysilane,di-n-octyldimethoxysilane, di-n-octyldiethoxysilane,di-n-cyclohexyldimethoxysilane, di-n-cyclohexyldiethoxysilane,diphenyldimethoxysilane and diphenyldiethoxysilane.

Particular preference is given to methyltrimethoxysilane,methyltriethoxysilane, ethyltrimethoxysilane and ethyltriethoxysilane.In one particular aspect of the present invention, the proportion orthese particularly preferred alkyltrialkoxysilanes is at least 80% byweight, in particular at least 90% by weight, based on the weight of thesilane compounds used.

In another aspect of the present invention, it is also possible to usesiloxane lacquers which comprise colloidally dispersed SiO₂ particles.These solutions may be obtained by the sol-gel process, in particular bycondensing tetraalkoxysilanes and/or tetrahalosilanes.

Aqueous coating compositions are usually prepared from theabovementioned silane compounds by hydrolyzing organosilicon compoundswith an amount of water sufficient for the hydrolysis process, i.e.>,0.5 mol of water per mole of the groups intended for hydrolysis, e.g.alkoxy groups, preferably with acid catalysis. Examples of acids whichmay be added are inorganic acids, such as hydrochloric acid, sulfuricacid, phosphoric acid, nitric acid, etc., or organic acids, such ascarboxylic acids, organic sulfonic acids, etc., or acidic ionexchangers, the pH during the hydrolysis reaction generally being from 2to 4.5, preferably 3.

Once the reactants have been combined, a rise in temperature isgenerally observed. In certain instances it can be necessary tointroduce external heat in order to start the reaction, for example byheating the mixture to 40-50° C. Care is generally taken to prevent thereaction temperature from exceeding 55° C. The reaction time isgenerally relatively short, and is usually less than one hour, forexample 45 min.

The silane compounds may be condensed to give polymers whoseweight-average molar mass M_(w) is generally from 100 to 20000 g/mol,from 200 to 10000 g/mol, and particularly preferably from 500 to 1500g/mol. An example of a method for determining this molar mass is NMRspectroscopy.

Examples of ways of terminating the condensation reaction are cooling totemperatures below 0° C., or increasing the pH, using suitable bases,e.g. alkali metal hydroxides or alkaline earth metal hydroxides.

For further operations, a portion of the water/alcohol mixture and ofthe volatile acids may be removed from the reaction mixture, for exampleby distillation.

Suitable organic solvents, e.g. alcohols, such as ethanol, methanol,isopropanol, butanol, ethers, such as diethyl ether, dioxane, ethers andesters of polyols, e.g. ethylene glycol, propylene glycol, or ethers andesters of these compounds, hydrocarbons, e.g. aromatic hydrocarbons,ketones, such as acetone, methyl ethyl ketone, may then be used toadjust the solids content to about 15-35% by weight, based on the totalweight of the mixture. Ethanol and/or 2-propanol is particularlypreferred as solvent.

It has also proven advantageous to add, to the coating compositions,solvents which normally solvate the plastic intended as substrate forcoating. In the case of polymethyl methacrylate (PMMA) as substrate, itis advisable to use, for example, an addition of solvents such astoluene, acetone, tetrahydrofuran in amounts making up from 2 to 20% byweight, based on the total weight of the compositions. The water contentis generally set at from 5 to 20% by weight, preferably from 11 to 15%by weight, based on the total weight of the compositions.

To improve storage stability, the pH of the aqueous siloxane lacquersmay be adjusted to the range from 3 to 6, preferably from 4.5 to 5.5.Additives, for example, may also be added for this purpose, inparticular propionamide, these being described in EP-A-0 073 911.

The siloxane lacquers which can be used according to the invention maycomprise curing catalysts, for example in the form of zinc compoundsand/or of other metal compounds, such as cobalt compounds, coppercompounds or calcium compounds, in particular their octoates ornaphthenates. The content of the curing catalysts is generally from 0.1to 2.5% by weight, especially from 0.2 to 2% by weight, based on theentire siloxane lacquer, but no resultant restriction is intended.Particular mention by way of example may be made of zinc naphthenate,zinc octoate, zinc acetate, zinc sulfate, etc.

The siloxane lacquers described above may be obtained commercially withthe trade names ®Acriplex 100 and ®Acriplex 180 SR from Röhm GmbH & Co.KG.

The siloxane lacquers described above may be applied to the plasticssubstrate using any known method. Among these are immersion processes,spray processes, doctoring, flow coating methods, and application byroller or roll.

The siloxane lacquers thus applied may generally be cured in arelatively short period, for example within from 2 to 6 hours, generallywithin from about 3 to 5 hours, and at comparatively low temperature,for example at from 70 to 110° C., preferably at about 80° C., to givecoatings with excellent scratch resistance and excellent adhesion.

The thickness of the layer of the siloxane coating (a) is relativelynon-critical. However, after curing this variable is generally withinthe range from 1 to 50 μm, preferably from 1.5 to 30 μm, particularlypreferably from 3 to 15 μm, with no intended resultant restriction. Thelayer thicknesses of the coatings (a) and/or (b) may be determined via ascanning electron micrograph (SEM).

In one particular aspect of the present invention, the polar componentof the surface energy is preferably at most 8 mN/m, particularlypreferably at most 6 mN/m after the curing of the first siloxane layer.

In one preferred embodiment of the present invention, the siliconcontent of the siloxane coating (a) after curing is at least 20% byweight, preferably at least 30% by weight, based on the total weight ofthe coating, with no intended resultant restriction. The carbon contentis preferably at most 36% by weight, in particular at most 25% byweight, based on the total weight of the coating. These contents may bedetermined by elemental analysis using the J. Liebig method or by atomicabsorption spectroscopy (AAS).

After the curing of the first siloxane coating, the surface is activatedby increasing the polar component of the surface energy of the curedsiloxane coating to a value of at least 10 mN/m. It is particularlypreferable for the polar component of the surface energy to be increasedto at least 15 mN/m.

The surface energy is determined by the Ownes-Wendt-Rabel & Kaelblemethod. For this, series of measurements are carried out using theBusscher standard series in which the test liquids used are water [ST72.1 mN/m], formamide [ST 56.9 mN/m, diiodomethane [ST 50.0 mN/m] andalpha-bromonaphthalene [ST 44.4 mN/m]. Measurement is carried out at 20°C. The surface tension and the polar and dispersion components of thesetest liquids are known, and these are used to calculate the surfaceenergy of the substrate.

The surface energy may be determined using a G40 contact anglemeasurement system from Krüss, Hamburg, the method being described inthe user manual for the G40 contact angle measurement system, 1993. Inrelation to the methods of calculation, reference may be made to A. W.Neumann, Über die Messmethodik zur Bestimmung grenzflächenenergetischerGröβen [Measurement methods for determining surface energy variables],Part I, Zeitschrift für Phys. Chem., Vol. 41, pp. 339-352 (1964), and A.W. Neumann, Über die Messmethodik zur Bestimmunggrenzflächenenergetischer Gröβen [Measurement methods for determiningsurface energy variables], Part II, Zeitschrift für Phys. Chem., Vol.43, pp. 71-83 (1964).

The various physical and chemical methods are suitable for activatingthe siloxane underlacquer. Among these are treatment of the surface bychemical methods, in particular by aqueous solutions, corona treatment,flame treatment, plasma treatment or atmospheric plasma treatment.Preference is given here to chemical methods and corona treatment.

The activation may take place by chemical means, by subjecting thesubstrate coated with siloxane underlacquer to treatment with,preferably liquid, reagents. It is preferable here that the incipientetching process affects only the uppermost atomic layers of the siloxanelacquer. In one particular aspect, the surface is treated with analkaline solution whose pH is at least 10, preferably at least 12.

For example, the siloxane-lacquer-coated substrate may be treated withan aqueous and/or alcoholic solution of alkali metal hydroxides.Preferred alcohols are methanol, ethanol, propanol and/or butanol. Theconcentration of the alkali metal hydroxides is preferably in the rangefrom 1 to 20% by weight, in particular from 2 to 10% by weight, based onthe weight of the etching solution. Particular alkali metals arelithium, sodium, potassium, rubidium and/or cesium. Among these,preference is given to sodium and/or potassium.

The period of exposure to the alkaline solution depends on the pH andmay therefore be within a wide range. However, a few minutes aregenerally sufficient. The period of exposure to the alkaline solution isparticularly preferably in the range from 30 seconds to 60 minutes, inparticular from 1 minute to 10 minutes. By way of example, this surfacetreatment may be terminated by neutral wash or addition of acids.

Any known method may be used to apply the alkaline solutions to thesiloxane coating. These methods have been described above.

The polar component of the surface energy may moreover be increased bycorona treatment. By way of example, this method is described in EP-A-1180 426. The treatment period depends on the energy used and ispreferably in the range from 1 to 20 seconds, in particular from 2 to 5seconds. By way of example, a generator suitable for corona treatmentmay be purchased from Softal Electronic GmbH, Hamburg, and can beoperated in the high-frequency range at from 20 to 30 kHz (generator3005).

After the activation of the first siloxane layer, in which nophotocatalytic content is present, a second layer comprising TiO₂particles is applied.

The lacquer for producing the second layer may be substantiallyidentical with the first siloxane lacquer, but it is necessary tointroduce photocatalytic TiO₂ particles.

By way of example, a lacquer of this type may be produced by mixing asiloxane lacquer described above with an aqueous and/or alcoholiccomposition comprising TiO₂ particles.

Particularly suitable coating compositions are in particular those whichcomprise colloidally dispersed SiO₂ particles. These particlespreferably have the same size as the TiO₂ particles described below.Dispersions of this type may be produced by the sol-gel process, inparticular condensing tetraalkoxysilanes and/or tetrahalosilanes.

Compositions of this type comprising TiO₂ particles are known, interalia, from EP-A-0 826 663, EP-A-0 850 203 and EP-1 022 318. Compositionsof this type may also, by way of example, be obtained commercially fromShowa Denko Kabushiki Kaisha, Japan with the trade name NTB 30A or fromToto Ltd., Japan.

The TiO₂ particles are photocatalytic. At least some of the TiO₂particles are therefore in the brookite and/or anatase form. Theparticle size is non-critical, but the transparency depends on theparticle size. The size of the particles is preferably at most 300 nmand in particular in the range from 1 to 200 nm, preferably from 1 to 50nm.

The second layer, comprising TiO₂ particles, may be applied and cured bythe abovementioned methods.

In one particular embodiment, the amount of the TiO₂ particles presentin the second coating is from 0.01 to 90% by weight, preferably from 0.1to 75% by weight, based on the total weight of the second coating aftercuring.

The thickness of the siloxane coating (b) comprising the TiO₂ particlesis likewise non-critical. After curing this thickness is generally inthe range from 0.05 to 2 μm, preferably from 0.1 to 1 μm.

In one particular embodiment of the plastics article, the total layerthickness of coatings (a) and (b) after curing is in the range from 2 to30 μm, in particular from 3 to 15 μm.

The plastics articles of the present invention, provided with aphotocatalytic coating, have high scrub resistance. The scrub resistanceto DIN 53778 is preferably greater than or equal to 10 000 cycles, inparticular greater than or equal to 15 000 cycles and particularlypreferably greater than or equal to 20 000 cycles.

In one particular aspect of the present invention, the plastics articleis transparent, the transparency τ_(D65/10) to DIN 5033 being at least70%, preferably at least 75%.

The plastics article preferably has a modulus of elasticity to ISO 527-2of at least 1000 MPa, in particular at least 1500 MPa, with no intendedresultant restriction.

The plastics articles of the invention are generally very resistant toweathering. For example, the weathering resistance to DIN 53387(Xenotest) is at least 5000 hours.

Even after extended UV irradiation for more than 5000 hours, theyellowness index to DIN 6167 (D65/10) of preferred plastics articles issmaller than or equal to 8, preferably smaller than or equal to 5, withno intended resultant restriction.

By way of example, the plastics articles of the present invention may beused in the building sector, in particular for the production ofgreenhouses or conservatories, or as a noise barrier.

The invention is illustrated in further detail below by inventiveexamples and comparative examples, but there is no intention that theinvention be restricted to these examples.

INVENTIVE EXAMPLE 1

PMMA sheets of size 150*350*3 mmm were provided with a scratch-resistantlacquer (®Acriplex 100 SR, Röhm GmbH & Co. KG), the layer thickness ofthe lacquer after curing being 7.5 μm.

After the curing of the lacquer, the polar component of the surfaceenergy was 5.5 mN/m. The surface was then treated for five minutes witha 5% KOH water/ethanol mixture (1:3 parts by weight), followed byneutral wash. The surface energy was determined using a G40 contactangle measurement system from Krüss, Hamburg, the test liquids usedcomprising water [ST 72.1 mN/m]], formamide [ST 56.9 mN/m, diiodomethane[ST 50.0 mN/m] and alpha-bromonaphthalene [ST 44.4 mN/m]. The polarcomponent of the surface energy was 15.3 mN/m.

After the activation, flow coating was used to apply a colloidalsolution comprising TiO₂ particles and comprising SiO₂ particles (3:1mixture of NTB 30A (TiO₂) with NTB 30B (SiO₂) obtainable fromShowa-Denko). The flow of the lacquer and the adhesion were good. Theresultant coating was cured for three hours at 80° C.

A Gardner M 105/A wet scrub tester was used for the scratch resistanceof the coating in the DIN 53778 wet scrub test. The value determined was20 000 cycles.

INVENTIVE EXAMPLE 2

Inventive Example 1 was in essence repeated, but NaOH was used insteadof KOH. The polar component of the surface energy was 12.8 mN/m.

The flow and the adhesion of the second coating was likewise good, andthe scratch resistance determined was 15 000 cycles.

INVENTIVE EXAMPLE 3

Inventive Example 1 was in essence repeated, but the first coating wasactivated by Corona treatment. Here, the sheet was passed four times at1 m/min through a Corona system (Softal Electronic GmbH, Hamburg,high-frequency range at from 20 to 30 kHz).

The flow and the adhesion of the second coating was likewise good, andthe scratch resistance determined was 12 000 cycles.

COMPARATIVE EXAMPLE 1

Inventive Example 1 was in essence repeated, but H₃PO₄ was used insteadof KOH. The polar component of the surface energy was 6.5 mN/m.

The flow and the adhesion of the second coating was so poor as toprevent any determination of scratch resistance.

COMPARATIVE EXAMPLE 2

Inventive Example 1 was in essence repeated, but no activation tookplace. The polar component of the surface energy was 5.5 mN/m.

The flow and the adhesion of the second coating was so poor as toprevent any determination of scratch resistance.

COMPARATIVE EXAMPLE 3

Inventive Example 1 was in essence repeated, but the first coating wassubjected to incomplete curing. The curing times were from 0.5 to 2.0hours at 80° C.

The flow of the coating solution on the partially cured layer wasunsatisfactory, and curing of the second layer gave a mechanicallyunstable coating which now lacked scratch resistance and which could bedamaged even by rubbing with a cloth. Determination of scratchresistance was not possible.

1. A self-cleaning plastics article produced by a process comprising:(i) applying a siloxane coating (a) to a plastic substrate, (ii) curingthe siloxane coating (a) to obtain a cured siloxane coating, (iii)increasing the polar component of the surface energy of the curedsiloxane coating to a value of at least 10 mN/in to obtain a polarcoating, (iv) applying a coating (b) comprising photocatalytic TiO₂particles to the polar coating, and (v) curing the coating (b) to obtainthe self-cleaning plastics article.
 2. The plastics article according toclaim 1, wherein the plastic substrate comprises at least one polymerselected from the group consisting of cycloolefin copolymers,polyethylene terephthalates, polycarbonates, and poly(meth)acrylates. 3.The plastics article according to claim 2, wherein the polymer ispolymethyl methacrylate.
 4. The plastics article according to claim 1,wherein the plastic substrate has an impact strength of at least 10kJ/m² to ISO 179/1.
 5. The plastics article according to claim 1,wherein the plastic substrate has a thickness in the range from 1 mm to200 mm.
 6. The plastics article according to claim 1, wherein thesiloxane coating comprises at least 80% by weight ofalkyltrialkoxysilanes, based on the content of condensable silanes. 7.The plastics article according to claim 1, wherein the siloxane coatingcomprises condensable polysiloxanes whose molar mass is in the rangefrom 500 to 1500 g/mol.
 8. The plastics article according to claim 1,wherein the proportion of silicon in the siloxane coating (a) is atleast 30% by weight, based on the total weight of the coating.
 9. Theplastics article according to claim 1, wherein the polar component ofthe surface energy of the siloxane coating (a) is lowered by curing to avalue smaller than or equal to 6 mN/m before said increasing the polarcomponent of the surface energy to at least 10 mN/m.
 10. The plasticsarticle according to claim 1, wherein said increasing the polarcomponent of the surface energy of the cured siloxane coating comprisestreating with alcoholic potassium hydroxide solution.
 11. The plasticsarticle according to claim 1, wherein the TiO₂ particles have a size inthe range from 1 nm to 300 nm.
 12. The plastics article according toclaim 1, wherein the coating (b) comprises from 0.01 to 90% by weight ofthe TiO₂ particles, based on the total weight of the coating (b) aftercuring.
 13. The plastics article according to claim 1, wherein the layerthickness of the siloxane coating (a) after curing is in the range from1.5 to 30 μm.
 14. The plastics article according to claim 1, wherein thelayer thickness of the coating (b) after curing is in the range from0.01 to 2 μm.
 15. The plastics article according to claim 1, wherein thelayer thickness of the coatings (a) and (b) after curing is in the rangefrom 3 to 15 μm.
 16. The plastics article according to claim 1, whereinthe scrub resistance of the plastics article according to DIN 53778 isat least 15,000.
 17. The plastics article according to claim 1, whereinthe plastics article has a modulus of elasticity according to ISO 527-2of at least 1500 MPa.
 18. The plastics article according to claim 1,wherein the plastics article has a weathering resistance according toDIN 53 387 of at least 5000 hours.
 19. The plastics article according toclaim 1, wherein the plastics article has a transparency according toDIN 5033 of at least 70%.
 20. The plastics article according to claim 1,wherein the plastics article has a yellowness index smaller than orequal to 5 after 5000 hours of UV irradiation.
 21. The plastics articleaccording to claim 1, wherein the polar component of the surface energyof the cured siloxane coating is increased to a value of at least 15mN/m.
 22. The plastics article according to claim 1, wherein the polarcomponent of the surface energy of the cured siloxane coating isincreased by corona treatment.
 23. The plastics article according toclaim 1, wherein increasing the polar component of the surface energy ofthe cured siloxane coating includes treating the cured siloxane coatingwith an alkaline solution having a pH of at least
 10. 24. The plasticsarticle according to claim 1, having a wet scrub value of at least20,000 cycles according to DIN
 53778. 25. A process for producing aself-cleaning plastics article, comprising: (i) applying a siloxanecoating (a) to a plastic substrate, (ii) curing the siloxane coating (a)to obtain a cured siloxane coating, (iii) increasing the polar componentof the surface energy of the cured siloxane coating to a value of atleast 10 mN/m to obtain a polar coating, (iv) applying a coating (b)comprising photocatalytic TiO₂ particles to the polar coating, and (v)curing the coating (b) to obtain the self-cleaning plastics article.